Highly sensitive electronic components and printed circuit board assemblies are increasingly being packaged using plastic materials with an antistatic finish. For example, both interdepartmental transport of printed circuit board assemblies at the manufacturer and transport to the customer use films and film laminates with an antistatic finish as packaging materials.
These films are very frequently heat-sealable films made from polyethylene or laminates of polyester and polyethylene films. The polyethylene film layers are often heat-sealed to one another at temperatures between 140.degree. C. and 200.degree. C. allowing packaging bags to be produced. These bags are usually provided with an antistatic finish either by coating the outside and inside with chemical antistatics, for example, quaternary ammonium salts, sterically hindered amines, and the like, or by treating or filling the polyethylene film in bulk early during production, such as during extrusion, with chemical antistatics, conductive carbon black and/or graphite or metal fibers or particles.
Films which are usually not themselves heat-sealable, such as, for example, biaxially stretched polyester and polypropylene films, can be laminated with heat-sealable films, for example, polyethylene, containing an antistatic finish. The non-heat-sealable film layers are optionally provided with antistatic coatings, for example, in the form of lacquers containing chemical antistatics or in the form of vacuum coatings with metals or metal oxides, before or after lamination. It is also possible to provide the non-heat-sealable films with hot-melt adhesive coatings in thicknesses of up to a maximum of a few microns, which are themselves then again provided with an antistatic finish, usually by means of addition of chemical antistatics.
Disadvantages in the use of fillers, such as, for example, conductive carbon black/graphite and/or metal fibers/particles, is the considerably reduced transparency of the bags and in the case of conductive carbon black possible abrasion, which can cause defects, for example short-circuits, in the packaged electronic components.
When chemical antistatics are used, the frequently corrosive action of these additives on metals, the usually inadequate antistatic action (usual surface resistances greater than 10.sup.10 ohm) and the generally high dependence of the antistatic action on the ambient atmospheric humidity are disadvantageous.
Since the beginning of the 1970s, there has been worldwide interest in the synthesis of "intrinsically" electroconductive polymers. These are polymer materials which, without addition of electroconductive substances such as metal powders or fibers, conductive carbon black or the like, have an inherent conductivity. Examples of such polymers are polyacetylene, polypyrrole, polythiophene, polyaniline, polyparaphenylene, polyphenylene sulfide, and the like. However, polyconjugated bond systems of this type are only electroconductive in the so-called "doped state" i.e., they must be converted into a conductive state by an electrochemical or chemical reaction using an oxidant or reducing agent. In the doped state, however, the above-mentioned materials are all insoluble and infusible, i.e., are unsuitable for further processing.
Until a few years ago, there were only a few concrete potential applications of intrinsically electroconductive polymers. A further disadvantage was the low stability of the materials, in particular, in humid atmospheres.
In order to obtain processable electroconductive polymers, soluble, intrinsically electroconductive polymers were developed (cf. R. L. Elsenbaumer, K. Y. Jen and R. Oboodi, Synth. Met. 15 (1986) 169). In particular, doped polyalkoxythiophenes synthesized by electrochemical methods being distinguished by high stability were described by M. Feldhues et al., Synth. Met. 28 (1989) C487. These polyalkoxythiophenes are, in doped form, sparingly soluble in organic aprotic solvents, such as toluene, tetrahydrofuran, acetonitrile, dimethylformamide or N-methylpyrrolidone, and are therefore suitable as base materials for the electroconductive and/or antistatic coating of substrates. See EP-A-0 328 981, EP-A-0 257 573 and EP-A-0 328 982.
In addition, coatings can also be produced by means of aqueous or non-aqueous dispersions based on intrinsically conductive polymers, for example, dispersions based on conductive polyaniline, see DE-A-38 34 526.
However, the coating of heat-sealable films and film laminates for the production, for example, of packaging bags with intrinsically electroconductive polymers entails the risk that the heat sealability of the coated films decreases considerably as a consequence of the coating and is no longer sufficient for the desired application.
DE-A-38 38 652 and EP-A-0 307 683 describe a process for the antistatic finishing of hot-melt adhesive coatings on plastic moldings. To this end, the hot-melt adhesive coatings, for example, fusible, high-molecular-weight ethylene-vinyl acetate copolymers and mixtures thereof with resins, waxes or paraffins, and copolymers of vinyl chloride and of vinylidene chloride, or copolymers of vinyl acetate and polymethacrylates, polyurethanes and polyesters, are coated with a solution or dispersion, if desired containing binders, of an electroconductive compound, for example, an electroconductive, finely divided solid. Electroconductive substances or compounds of this type may also be, inter alia, intrinsically electroconductive polyheteroaromatic compounds, conductive carbon blacks, metal oxides or metal powders. Additional binders may serve to improve the adhesive bond by means of heat sealing and/or high-frequency (HF) welding.
However, it is generally known that coatings of this type which contain infusible electroconductive materials, such as, for example, doped, intrinsically conductive polymers, and are not applied to hot-melt adhesive coatings, but instead directly to plastics, in particular films, which are usually themselves heat-sealable, thermally weldable, or HF- or ultrasound-weldable, greatly diminish the heat-sealing properties of these plastics. For this reason, for example, heat-sealable polyethylene films are today usually rendered antistatic by addition of small amounts of chemical antistatics such as quaternary ammonium salts, alkylsulfonates, alkyl sulfates, alkyl phosphates, fatty acid esters, fatty acid ethers, and the like. These antistatics attract ambient moisture due to their hydrophilic properties and thus generate ions and conductivity. However, the surface resistance achieved in this way is usually very high, on the order of .gtoreq.10.sup.10 ohm, many of these substances have a corrosive action on metal parts, and the antistatic action is humidity-independent. However, antistatics of this type only reduce the heat-sealability slightly.
EP-A-0 292 905 and EP-A-0 328 981 which are both hereby incorporated by reference, describe coating compositions for the antistatic finishing of films, which compositions contain one or more binders and from 10% to 100% by weight of an intrinsically electroconductive, infusible, electrochemically synthesized oligomer which, in the oxidized form, is completely soluble in polar aprotic solvents at room temperature and has been obtained, inter alia, from structural units of the formula ##STR2## in which R.sup.1 is a C.sub.1 - to C.sub.12 -alkoxy group (EP-A-0 292 905) or a C.sub.6 - to C.sub.30 -alkoxy group (EP-A-0 328 981). These electroconductive oligomers contain, in their oxidized form, an appropriate number of anions, preferably anions of the conductive salt employed in the electrolysis preparation process, in order to compensate for the positive charges on the oligomer chain. Examples of the anions are BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-, SbCl.sub.6.sup.-, FeCl.sub.4.sup.-, [Fe(CN).sub.6 ].sup.3-, and the like.
DE-U-88 14 712.6 describes multilayer films which are provided with layers of these conductive polymers or oligomers. Heat-sealing properties of such multilayer films, inter alia, polyester/polyethylene systems are not described therein.
Kunststoffe 82 (1992), 22 describes the coating of a polyethylene terephthalate film with polyethoxythiophene (PEOT). The coating was carried out by intaglio printing with a lacquer having a solids content of from 4% to 8% at an application rate of from 1 to 4 g/m.sup.2, so that the dry thickness of this antistatic coating was at least 100 nm.
Experiments have now shown that the above-described coating compositions applied in dry thicknesses of 100 nm or more, generally reduce the heat-sealability of the previously heat-sealable films so much that heat-sealing or welding by means of heat, ultrasound or high results.
Accordingly, if it is desired to utilize the heat-sealing properties of a film of this type, care must be taken that no coating is applied to the film edges to be heat-sealed or welded, which requires a very complex procedure since a separate coating device, for example in the form of especially engraved intaglio printing rollers, must be available for every film width that is called for.